With the advance in the reduction in the vehicle weight and the increase in the output of the vehicle engine, the coil springs used for wheel suspension systems and the valve springs used in internal combustion engines have come to be required to be able to withstand increasingly higher stresses, and, in particular, such springs are desired to be resistant to permanent set and fatigue.
Breakage of a coil spring due to fatigue typically occurs through the generation and propagation of cracks which typically start from defects such as inclusions on the surface or in the interior of the material of the coil spring. However, with the progress in the steel making technology, the occurrence of fatigue breakage due to inclusion of foreign matters in the material has become very rare. As a measure to further enhance the fatigue strength of coil springs, it has been proposed to carbonitride and strengthen the material surface. In particular, it has been proposed to achieve a nitriding depth of 70 .mu.m or more so that a high compressive residual stress may be produced on the surface (refer to Japanese patent laid open publications (kokai) Nos. 7-11422, and 5-156351).
However, a carbonitrization process of a relatively long time duration is required to achieve a nitriding depth of 70 .mu.m or more, and, therefore, even though the fatigue strength may be improved, the hardness of the interior of the material may be reduced so that the resistance of the material to permanent set could be impaired. This will be the case particularly when the load stress is high, and the material could lose its load bearing capability in use.
It is also known to set the upper limit of the surface roughness of the material so that the notch sensitivity may not impair the resistance of the material to permanent set (refer to Japanese patent laid open publication (kokai) No. 7-11422). The notch sensitivity tends to increase with the increase in the steepness of the irregularities or the surface roughness, and such an increase in the notch sensitivity leads to the reduction in the service life of the spring material.